PHARMACOLOGY AND THERAPEUTICS The British Pharmacopeia contains some twenty-five mercurial preparations, including those of calomel (q.v.). Only the useful preparations will be mentioned here. Free mercury is contained in Hydrargyrum cum Creta, or "grey powder," which consists of one part of mercury to two of prepared chalk. The dose is 1-5 gr., and the preparation is usually employed for children. The Pilula Hydrargyri, or "blue pill," contains one part of mercury in three, and the dose is 4-8 gr. It is usually employed for adults. There are also five preparations of free mercury for external use. Of these the most useful is the Unguentum Hydrargyri, "or blue ointment," which contains one part of mercury in two. Weaker ointments are also prepared from the red and the yellow forms of mercuric oxide. The perchloride of mercury or corrosive sub limate is therapeutically the most important salt of mercury. The dose is -1---A_ gr. It is incompatible with alkalis, alkaline carbonates, potassium iodide, albumin and many other sub stances, and should therefore be prescribed alone. It is de composed by impure water, and distilled water is therefore used in making the Liquor Hydrargyri Perchloridi, in which form it is usually prescribed. This contains half a grain of the perchloride to the fluid ounce and its dose is 3o-6o minims. The perchloride is also compounded with lime-water to form the Lotio Hydrargyri Flava, or "yellow wash," which contains two grains of the salt to the fluid ounce. Mercuric iodide is an equally potent salt and has come into wide use. It has the same dose as the perchloride and is largely prescribed in the Liquor Arsenii et Hydrargyri Iodidi, or Donovan's solution, which contains 1% of arsenious iodide and 1% of mercuric iodide, the dose being 5-2o minims. An ointment widely used is prepared from the mercur-ammonium chloride (Unguentum Hydrargyri ammoniatum) of which it con tains one part in ten. It is known as "white precipitate ointment." In discussing the pharmacology of mercury and its compounds, it is of the first importance to observe that metallic mercury is inert as such, and that the same may practically be said of mer curous salts generally. Both mercury itself and mercurous salts
tend to be converted in the body into mercuric salts, to which the action is due. When metallic mercury is triturated or ex posed to air it is partly oxidized, the first stage of its transforma tion to an active condition being thus reached.
Metallic mercury can be absorbed by the skin, passing in minute globules through the ducts of the sweat-glands. The mercury contained in "blue ointment" is certainly thus absorbed, actually circulating in the blood in a very different form, as described below. There is no local action on the skin. The mercuric salts, and especially the chloride and iodide, are probably the most powerful of all known antiseptics. One part of the perchloride in 500,000 will prevent the growth of anthrax bacilli, and one part in 2,000—the strength commonly employed in surgery— kills all known bacteria. The action is apparently specific and not due to the fact that perchloride of mercury precipitates albumin, including the albuminous bodies of bacteria, for the iodide is still more powerful as a germicide, though it does not coagulate albumin. These salts cannot be employed for sterilizing metallic instruments, which they tarnish. As these drugs are essentially poisons they must be used with the greatest care in surgical practice, and as they are particularly deleterious to the secreting structure of the kidney they must not be employed as antiseptics in diseases where renal inflammation is already present or probable. They are therefore contra-indicated for application to the throat in scarlet-fever or to the uterus in eclampsia. The stronger mercurial ointments kill cutaneous parasites and also possess some degree of antipruritic action, especially when the cause of the itching is somewhat obscure. Mercuric salts, when in strong solution, are caustic. It is important to observe that the volatility of metallic mercury and many of its compounds causes their absorption by the lungs even when no such effect is intended to follow their external application. This fact explains the occurrence of chronic mercurial poisoning in certain trades.